Project Summary: An estimated 50% of HIV+ patients still exhibits central nervous system (CNS) viral infection, with 30% of patients progressing to some form of HIV-associated neurocognitive disorder (HAND) despite combined anti- retroviral therapy (cART). In the previously funded NIH R01 (NS063897-01A2) project ? Viral evolution in peripheral macrophages and brain during progression to AIDS ? we used the SIV-infected macaque model of neuroAIDS to show that in untreated animals SIV can enter the CNS multiple times, as early as 10 days post infection, throughout the course of the disease. SIV subpopulations infecting the brain are evolutionarily related to viral strains infecting myeloid cells in peripheral tissues, such as bone marrow and lung, which accumulate in the meninges and choroid plexus in early infection, and in the perivascular space and SIV-associated encephalitis (SIVE) lesions in late infection. Moreover, we found evidence that ongoing evolution in peripheral tissues leads, late in infection, to the emergence of viral lineages adapted to enter and replicate in the CNS microenvironment that may be linked to SIVE onset. However, the impact of cART on the timing and mode of entry of virus into the CNS has yet to be analyzed, which is crucial to explain why HAND is present even in virally suppressed patients. The present proposal for a competitive renewal seeks to extend the studies of the original project by characterizing the evolutionary behavior of the virus leading up to CNS infection and subsequent regulation of CNS-specific viral and host genes expression in the presence of cART. We seek to evaluate, in particular, peripheral blood monocytes and monocyte/macrophage rich tissues as potential source of CNS virus during cART, and the contribution of persisting CNS (and peripheral tissues) virus to viral rebound after therapy interruption. Two specific aims are proposed: Specific Aim 1 ? Determine the impact of early cART on viral evolutionary patterns, as well as viral and host gene expression patterns, associated with viral entry and replication in the CNS of SIV-infected rhesus macaques; Specific Aim 2 ? Determine relative contribution of SIV-infected macrophage subsets in the CNS and peripheral tissues to low-level viremia during cART and viral rebound following cART interruption. The elucidation of viral evolutionary patterns and the contribution of specific infected tissues/cell populations to CNS infection, in the presence or subsequent absence of cART, would be highly beneficial to future studies designed to adjust current therapeutic strategies toward the prevention and elimination of neuroAIDS, and formation of the CNS reservoir, which is a necessary step for the development of an HIV cure.